UNITED STATES - AUGUST 8: The border security fence stretches over the hills of Nogales, Ariz., separating the U.S. and Mexico. (Photo By Bill Clark/CQ Roll Call)

What would happen if the United States suddenly stopped building walls and instead flung open its borders, not unlike the European Union has done among the member countries of the common market? Conservatives malign the notion and liberals, even radical ones, haven't exactly embraced the "open borders" concept.

But the idea isn't as radical as it may seem. For most of its history, the United States has had, for all practical purposes, open borders, according to University of San Francisco law professor Bill Hing.

"Really, the United States was an open-border situation, worldwide, up through the early 1900s -- except for Asians," Hing told The Huffington Post. "There were Asian-exclusion laws. But if you put that aside, it was open borders for the rest of the world."

Here are 16 reasons why opening our borders makes more sense than militarizing them. Let us know what you think in the comments.

Politicians, journalists and activists of every ideology refer to the U.S. immigration system as "broken." If what we're doing doesn't work, why not try something different?

2Because it's what we used to do.

A Photograph of an Immigration Officer on Ellis Island, New York circa 1880. (Getty)

There's a notion that in the 19th and early 20th centuries, Europeans immigrated to the U.S. legally. But "legally" meant something very different than it does now. At that time, the United States accepted practically everyone who showed up, with few restrictions other than the 1882 Chinese Exclusion Act and a brief health examination.

Rich individuals park their fortunes in foreign countries to avoid taxes. People from wealthy families cross borders with ease. So why put such harsh restrictions on people who immigrate to work for paychecks?

4Because if capital and goods can flow across borders, why not labor?

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The North American Free Trade Agreement made it easier for business owners to invest in Mexico and for goods to flow freely across the U.S.-Mexico border. But the millions of Mexicans put out of work by these changes weren't permitted to cross the border in search of jobs created here.

"If you think about it, corporations have open borders," Arturo Carmona, executive director of the grassroots organization Presente.org, told HuffPost. "But when you think about workers' rights, family reunification -- you have closed borders."

5Because undocumented immigrants are the victims of legislation that is, frankly, dumb.

A man looks out toward the U.S. from the Mexican side of the border fence. (Getty)

The undocumented population began skyrocketing in the 1960s when the United States started restricting the number of immigrant visas for Mexicans. In 1977, Congress capped the number of such visas for Mexican workers at 20,000, a number wildly out of sync with labor demand. Subsequent revisions to immigration law consistently disregarded demand for Mexican, and to a lesser degree, Central American labor.

6Because immigration improves the economy.

Construction worker Sebastian Martinez of Mexico works at a construction site. (AP)

From all the controversy surrounding the issue, you might think that immigration was an economic evil that the U.S. is forced by circumstance to put up with. In fact, the consensus among economists is that immigration -- both legal and illegal -- is good for the overall economy. While competition from undocumented workers and new arrivals pushes wages down in some sectors, the net effect is positive.

7Because Europe opened many of its borders, and the sky didn't fall.

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As Bill Hing points out, when the European Union was created, effectively allowing the free movement of EU citizens across the common market's borders, a funny thing happened. Countries once known for their high output of immigrants, like Spain, Portugal and Ireland, became immigrant-receiving countries -- a pattern that held until the worldwide economic crisis in 2007.

"Why?" says Hing. "Because there was huge investment in their economies. If we approach immigration the way they did in the EU [...] you actually will not see a hysterical flood of migrants across the border. But I do think it needs to be coupled with serious investment in poor areas of Mexico."

8Because it makes humanitarian sense.

Central American migrants climb on a northbound train in Ixtepec, Mexico. (AP)

Some 60,000 unaccompanied minors have crossed the border illegally in the past year, most of them seeking refuge from poverty and violence in their home countries. That's to say nothing of the thousands of Mexicans applying for asylum to escape the country's drug war or the millions of others simply looking for work.

Americans are proud of their country's history of harboring Irish families fleeing famine in the late 19th century, as well as Jewish refugees from World War II. Why not refugees from Mexico and Central America?

9Because the U.S. is responsible for creating the conditions that cause illegal immigration.

From overthrowing left-wing governments during the Cold War, to financing military forces that massacred civilians, to pushing economic policies that put Mexican farmers out of work by the millions, the United States has helped create a lot of the conditions that have resulted in mass migration from Latin America. It would logically follow that the U.S., which has reaped economic benefits from this mayhem, should allow immigrants to stay.

10Because it allows people to go back.

A migrant farm worker presents his documentation. (AP)

Most people only think of undocumented immigrants as coming into the United States, but for most of the United States' history, Mexican and Central American workers crossed into the country for seasonal agricultural labor and then returned home. Some people who work in other industries come to the U.S. to raise capital and return home to start a business. With the militarization of the border, returning has become so difficult that most people simply stay.

11Because fewer people will die crossing the border.

A van of migrants travel past a memorial asking "How many more have to die?" (AP)

Putting up the border wall in high-traffic areas has simply pushed crossings into the desert, where more people die.

Neither party can boast a record of fiscal responsibility when it comes to the border. The U.S. spends more on border enforcement than on all other federal law enforcement agencies combined.

13Because border enforcement punishes the wrong people.

A guard escorts an immigrant detainee from his 'segregation cell.' (AP)

Much of the resentment against undocumented immigrants is rooted in the inaccurate belief that they damage the economy, and the semi-accurate belief that they depress wages in some sectors.

But that's unfair. Wages for American workers have been stagnant since the 1970s, well before the latest wave of mass migration. And many economists say the destruction of high-paying jobs in sectors like manufacturing has far more to do with globalization, the lowering of tariffs that protected U.S. industries and companies moving overseas. Low-skilled workers rightly angry about their sagging wages and lack of job opportunities have more of a bone to pick with business leaders and elected officials than they do with their fellow workers.

From tech to agriculture, U.S. employers are clamoring for the government to increase the number of immigrant visas to supply their industries with workers. Imagine what our economy would look like if people in Alabama couldn't move to take jobs in New York City, or if people in Louisiana couldn't work for a better future in Texas. That's more or less what our immigration policy currently looks like.

15Because our immigration system criminalizes people unnecessarily.

Immigrant inmates line up for breakfast at the Maricopa County Tent City jail. (Getty)

Illegal entry and illegal re-entry charges now make up the largest category of crimes on the federal docket, outnumbering drug offenses. If our borders were more open, there would be no need to incarcerate these people.

16Because our enforcement system is a piggy bank for the private prison industry.